This invention relates to a process for treating a liquid-wet polycarbonate membrane to improve its gas separation properties.
The use of polymeric membranes for gas separation is well known. A wide variety of polymers have been used for gas separation membranes, including cellulose esters, polyamides, polyimides, polysulfones, and polyolefins. An application of particular interest is membrane separation of oxygen and nitrogen from air. An enriched nitrogen stream obtained from air may be used for inert padding of flammable fluids or for food storage. An enriched oxygen stream obtained from air may be used for enhancing combustion or for increasing the efficiency of fermentation processes.
The membranes used for gas separations are preferably dry so that the most effective membrane separation performance can be achieved. However, many membranes are formed by the wet process, in which a solution of polymer, solvent(s) and optional non-solvent(s) is cast or extruded, the solvent(s) and optional non-solvents(s) optionally allowed to partially evaporate, followed by immersion in a coagulating liquid bath, often water, to gel and solidify the membrane while optionally extracting at least a portion of the solvent(s) and optional non-solvent(s). Thus, the membranes formed by the wet process are liquid-wet and preferably are dried prior to use for gas separation.
The art teaches that care must be taken during the drying process to maintain the physical structure of the membrane because structural changes such as pore collapse or crazing result in adverse membrane performance. The art discloses several techniques for drying a water-wet membrane fabricated from a cellulose ester so that the physical structure of the membrane is preserved. One such method is freeze drying. Another method involves sequentially contacting the membrane with polar and non-polar solvents. The purpose of the sequential solvent method is to sufficiently reduce the polymer-water interaction by replacing water with a non-polar solvent, thus lowering the surface tension, so that the membrane may be dried without an adverse impact on the structure of the membrane. The problem is that such techniques are expensive, time consuming, and generate large volumes of solvent for disposal. Furthermore, such techniques introduce sources of significant variation in membrane performance.
Polycarbonate membranes in particular have been discovered to possess good separation properties for gases, especially oxygen and nitrogen. Polycarbonate membranes formed by the wet process generally are porous or asymmetric, depending on the extrusion or casting conditions. Porous membranes may be used as supports for composite gas separation membranes. Composite membranes possess a thin, dense discriminating layer supported on a porous substructure of a different material. Asymmetric membranes possess a thin, dense discriminating layer supported on a porous substructure of the same material. The discriminating layer provides the membrane with gas separating capability. The membrane discriminating layer is preferably as thin as possible while still maintaining the ability to separate gases in order that the highest possible gas flux through the membrane may be achieved. POWADIR membranes may also be fabricated by the wet process. POWADIR membranes possess one or more discriminating regions capable of separating gases and one or more porous regions. An asymmetric membrane is a POWADIR membrane, but a POWADIR membrane is not necessarily asymmetric.
Polycarbonate membranes formed by the wet process may be directly dried in air. However, such polycarbonate membranes generally contain small amounts of residual solvent and non-solvent even after leaching which adversely affect the performance of the dried membranes. The presence of even small amounts of residual solvent and non-solvent in the dried membrane can result in reduced gas flux, reduced separation factor (selectivity), increased membrane performance variability, and increased compaction rate. An inexpensive, timely, and reproducible method of drying polycarbonate membranes which enhances separation properties through the removal of residual solvent and non-solvent prior to drying is needed. Such a method preferably does not employ toxic solvents which present problems with respect to disposal and exposure of personnel during the membrane fabrication process.
Furthermore, polycarbonate membranes formed by the wet process may possess a lower than optimal gas selectivity because of microscopic deficiencies in the membrane's morphological structure. For example, the discriminating layer may contain microscopic defects interrupting the continuity of the discriminating layer, resulting in a less than optimal gas selectivity, or the discriminating layer may not be "dense" enough, that is, the pores in the discriminating layer may not be sufficiently small so that the discriminating layer is capable of efficiently separating gases. Therefore, a process is also needed which results in increased gas selectivity through a modification of the membrane's morphological structure by "tightening" the discriminating layer with out producing a significant decrease in the gas flux through the membrane. Such a treatment process also preferably imparts to the membrane long term operating stability at high temperatures and pressures. That is, the membranes so treated preferably do not exhibit a significant compaction rate over time. Compaction rate consists of a significant loss of separation properties measured as a decrease in gas flux over time under given conditions of operating temperature and pressure.
A single process which results in a dry polycarbonate membrane with improved membrane separation properties through both removal of residual solvent and non-solvent and a modification of the membrane's morphological structure to substantially eliminate compaction during subsequent operation would be particularly advantageous.